I also got an error trying to put something in my calendar, but it didn't give me a report. Plus, it isn't giving me an error anymore. I'm running this server off of an old K6 366mhz computer for testing purposes.

I've already gone into the MTA tab and turned off DNS lookus and disabled all that other stuff in there.

wrt SEND_FAILURE

if you see a SEND_FAILURE, specifically with this text "not sent to any recipients because these failed addresses", it is time to check /opt/zimbra/log/zimbra.log (or /var/log/zimbra.log, depending on the release). The webapp (inside tomcat) contacted postfix (via JavaMail APIs), and issued a RCPT TO: command, and postfix reject the RCPT TO:. The reason why postfix rejected the RCPT TO: should be in the logs along with the exception stack trace for the SEND_FAILURE (follow the causing exception chain).

Usually when this happens, specially with a @gmail.com address (ie, a common and well known domain), I begin to wonder if you have reject_unknown_hostname (in admin console Global Settings/MTA tab) enabled and DNS disabled. If that is true postfix not be able to DNS lookup gmail.com and will reject the RCPT TO: command. I am not saying this is your problem - consider this an example of how this failure might occur.